In order that the user of a terminal device would hear a sound as optimal as possible as regards its strength and quality, the speaker of the device must usually be held against the ear as close as possible. In other words, normally the user experiences that he or she hears the best sound in respect of quality when the terminal is pressed tightly against the ear, in which case a kind of an acoustic, sealed space is created between the mobile station and the ear.
If the speaker is not pressed against the ear, a gap is created between the ear and the speaker via which the acoustic waves are leaking to the side. The more far off the speaker is kept from the user's ear, the bigger part of the acoustic waves generated by the speaker passes the ear. The sound pressure and sound volume sensed by the user is thus the smaller the bigger is the distance (in other words leakage) between the speaker and the ear.
In case the leakage is strong, the user will perceive changes in the quality of the sound reaching his or her ear, since low frequencies tend to weaken more than high frequencies. In this connection, the term leakage tolerance is introduced, which means the capability of the terminal device to maintain the aforementioned acoustic properties of sound, when the distance between the speaker and the ear changes and the aforementioned acoustic space changes as well.
The problem referred to above can be encountered especially in mobile stations and corresponding terminal devices, since the aforementioned devices are typically not pressed, and they cannot necessarily be pressed, tight against the ear. The improving of the leakage tolerance especially in small devices is problematic, since the devices typically have little space in order that the solutions improving the leakage tolerance could be easily introduced. When the terminal devices are getting smaller and smaller in respect of their physical size, there is less and less space left in the device structure for different solutions improving the leakage tolerance.
The size of the acoustic structure space that is possibly left inside the device structure affects the amount of the so-called acoustic impedance of the device. It would be preferable to get the acoustic impedance, i.e. acoustic output impedance of the speaker, as small as possible. When the aforementioned output impedance is small, the distance of the device or speaker from the ear does not have such a big effect on the quality of sound sensed by the user.
The objective of the invention is to overcome the aforementioned disadvantage and to disclose a solution which enables one to achieve a leakage tolerance better than before for the mobile station and the like devices.